An integrated circuity is formed in and on a wafer in semiconductor manufacturing processes. Forming an integrated circuit on a wafer involves a number of sub-steps such as thermal oxidation, masking, etching and doping. In the thermal oxidation sub-step, the wafers are exposed to ultra-pure oxygen under carefully controlled conditions to form a silicon dioxide film, for example, on the wafer surface. In the masking sub-step, a photoresist or light-sensitive film is applied to the wafer, an intense light is projected through a mask to expose the photoresist with the mask pattern, the exposed photoresist is removed, and the wafer is baked to harden the remaining photoresist pattern. In the etching sub-step, the wafer is exposed to a chemical solution or gas discharge to etch away or remove areas not covered by the hardened photoresist. In the doping sub-step, atoms with either one less or one more electron than silicon are introduced into the area exposed by the etching process to alter the electrical character of the silicon. These sub-steps are repeated for each layer. Most of or all of these processes require the controlled introduction of gases into a processing chamber, and mass flow controllers are used to control the same. Each chip on the wafer is finally tested after the remaining metals, films and layers have been deposited. Subsequently, the wafer is sliced into individual chips that are assembled into packages.
Semiconductor gases are used in the above-described manufacturing process, and include, but are not limited to gases which serve as precursors, etchants and dopants. These gases are applied to the semiconductor wafer in a processing chamber. Precursor gases provide a source of silicon atoms for the deposition of polycrystalline silicon, epitaxial silicon, silicon dioxide and silicon nitride film within the thermal oxidation step. Etchant gases provide fluorocarbons and other fluorinated materials that react with silicon, silicon dioxide and silicon nitride. Dopants provide a source of controllable impurities that modify the local electrical properties or characteristics of the semiconductor material. A reliable supply of high purity process gases is required for advanced semiconductor manufacturing. As the semiconductor industry moves to smaller feature sizes, a greater demand is placed on the control technologies to accurately and reliably deliver the semiconductor process gases.
Mass Flow Controllers (MFCs) are placed in an inflow line to control the delivery of the semiconductor process gas. Conventional MFCs have an iris-like restricted orifice for controlling flow, and deliver gas or other mass at a low velocity. This low velocity allows interfering feedback in the MFC; i.e. the pressure differentials occurring in the chamber travel back upstream through the gas and perturb the delivery velocity of the gas. Therefore, a problem associated with conventional MFCs is that they are dependent on the characteristics of the specific chamber into which the gas is being delivered, and require trial and error methods to find the proper valve position for delivering a desired flow of material into the chamber. An obvious drawback to this approach is that the experimentation is very time consuming.
Ultrasonic MFCs meter gas flowing through an orifice of a known size at a velocity higher than the speed of sound. The mass flow is controlled using a gated orifice by oscillating a gate between an opened and closed position with respect to the orifice. The amount of material delivered into the chamber is adjusted by adjusting the duty cycle of the oscillations; i.e. by adjusting the amount of time per oscillation period that the gate is opened rather than closed. Because pressure differentials can only travel through the gas at the speed of sound, pressure variations in the chamber do not travel upstream quickly enough to perturb the ultrasonic delivery velocity. Thus, ultrasonic MFCs have feed forward control as they are able to deliver exactly the desired amount of material into the chamber without being affected by any feedback from the chamber. However, a problem associated with ultrasonic MFCs is that control gates regulating the precision flow may fail by becoming stuck either in an opened position, a closed position, or in some position in between the opened and closed positions. And in the case of the above-described process for manufacturing semiconductors, this failure may not be detected for a considerable amount of time causing considerable losses in both processing time and resources.
Therefore, there is a need in the art to provide improved MFC which overcomes these problems.